This application is related to Japanese Patent Applications No. 2000-118394 filed on Apr. 19, 2000, and No. 2000-311142 filed on Oct. 11, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a heat-pump water heater that heats water using a super-critical (transcritical) heat pump cycle as a heating source.
2. Description of Related Art
In a conventional heat-pump water heater, low-temperature water is heat-exchanged with high-temperature refrigerant in a water heat exchanger, and high-temperature water heated in the water heat exchanger is stored in a water tank to be supplied to a user after being temperature-adjusted. In the heat-pump water heater, a target temperature difference xcex94T between water flowing into the water heat exchanger and refrigerant discharged from the water heat exchanger is set, and high-pressure side refrigerant pressure of the heat pump cycle is controlled based on the target temperature difference xcex94T for increasing a cycle efficiency of the heat pump cycle. Generally, the high-pressure side refrigerant pressure is controlled by adjusting a valve opening degree of the expansion valve.
However, when the high-pressure side refrigerant pressure is controlled based on the target temperature difference xcex94T when the heat-pump water heater is used under a low temperature, a low-pressure side refrigerant pressure (e.g., evaporation pressure) of the heat pump cycle is decreased, and temperature of refrigerant discharged from a compressor may exceed a normal operation temperature area of the compressor.
On the other hand, when the high-pressure side refrigerant pressure of the heat pump cycle is increased due to an outside air increase, a water temperature increase, a rotation speed increase of the compressor or a deterioration of operation performance of the water heat exchanger, load of the compressor increases, and a normal operation of the heat pump cycle may be affected. In this case, when the rotation speed of the compressor is decreased for preventing the overload of the compressor, it is difficult to obtain a necessary heating capacity in the water heater only by controlling the valve opening degree of the expansion valve.
In view of the foregoing problems, it is an object of the present invention to provide a heat-pump fluid heater for heating a fluid (e.g., water) using a heat pump cycle as a heating source, in which a refrigerant temperature discharged from a compressor can be controlled in an operation temperature area even when the heat pump cycle is used under a low temperature.
It is an another object of the present invention to provide a heat-pump fluid heater which prevents a problem of a heat pump cycle due to a load increase of a compressor, while obtaining a desired water-heating capacity in a water supply system.
According to the present invention, in a heat pump fluid heater for heating a fluid (e.g., water) using a heat pump cycle as a heating source, a control unit for controlling operation of the heat pump cycle controls a high-pressure side refrigerant pressure from the compressor and before being decompressed in the heat pump cycle, so that a temperature difference between the fluid flowing into a heat exchanger and refrigerant discharged from the heat exchanger becomes a set target temperature difference. Further, the control unit has a detection member for detecting one of a refrigerant temperature and a physical amount relative to the refrigerant temperature discharged from the compressor, and the control unit changes the target temperature difference to be increased when a detection value of the detection member is more than a predetermined value. When the target temperature difference is changed and becomes larger, a heat-exchanging efficiency of the heat exchanger is decreased, and a heat-exchanging amount in the heat exchanger is reduced. That is, in this case, because the refrigerant pressure discharged from the compressor is controlled to be decreased, the refrigerant temperature discharged from the compressor is decreased. Accordingly, even when the heat pump cycle is used under a low temperature condition, the refrigerant temperature discharged from the compressor can be controlled in an operation temperature area.
Preferably, the control unit sets the target temperature difference larger as a low-pressure side refrigerant pressure after being decompressed in the heat pump cycle becomes lower, when the low-pressure side refrigerant pressure in the heat pump cycle is lower than a predetermined pressure. When the low-pressure side refrigerant pressure (e.g., evaporation pressure) of the heat pump cycle is decreased due to a decrease of outside air temperature, for example, load of the compressor is increased and refrigerant temperature discharged from the compressor is increased. Accordingly, by setting the target temperature difference larger as the low-pressure side refrigerant pressure becomes lower, it can effectively restrict the refrigerant temperature discharged from the compressor from being increased.
On the other hand, the control unit determines whether or not a load of the compressor is excessive, and the control unit changes the target temperature difference to be increased to a value when it is determined that the load of the compressor is excessive. In this case, when the target temperature difference is made larger, the compressor continuously operates with a relatively lower high-pressure. Accordingly, is can prevent a problem of a heat pump cycle due to an increased load of the compressor. Further, when the fluid is water in a hot water supply system, a desired water-heating capacity can be obtained.
According to the present invention, the control unit has a temperature detection sensor for detecting a refrigerant temperature discharged from the compressor, and the control unit controls a high-pressure side refrigerant pressure from the compressor and before being decompressed in the heat pump cycle, so that a temperature difference between the fluid flowing into the heat exchanger and refrigerant discharged from the heat exchanger becomes a target temperature difference when the refrigerant temperature detected by the temperature sensor is lower than a predetermined temperature. On the other hand, when the refrigerant temperature detected by the temperature sensor is higher than the predetermined temperature, the control unit controls the high-pressure side refrigerant pressure of the heat pump cycle so that the refrigerant temperature detected by the temperature sensor becomes lower than the predetermined temperature. Accordingly, the refrigerant temperature discharged from the compressor can be directly controlled without changing the target temperature difference.